Suppression of ionization probability due to Rabi oscillations in the resonance two-photon ionization of He by EUV free-electron lasers

The two-photon resonance ionization probability of atoms in strong extreme-ultraviolet free-electron laser (EUV FEL) pulses has been investigated by the model of time-dependent wave packet propagation of a light-coupled multilevel atom. Under the simulation within the model assuming single-mode FEL pulses, the ionization probability Pion has shown characteristic dependences on the scaled coupling parameter Ugi between two levels of the ground (g) and intermediate (i) resonance states, namely, Pion∝(Ugi)n, with n being equal to ∼2, less than 1, and ∼1 for the small, medium, and large Ugi regimes, respectively. This power dependence of the ionization probability has been interpreted due to Rabi oscillations between g and i states. To compare with recent experimental results on the same condition, the multimode nature of self-amplitude spontaneous emission (SASE) FEL pulses has been managed in the simulation. Then, the recent experimental laser-power dependence of the two-photon resonance ionization of He [Sato et al., J. Phys. B 44, 161001 (2011)] has been well described by that for the large Ugi regime of the simulation, i.e., n∼ 1. Thus, the observed linear laser-power dependence has been rationalized as being caused by the strong Rabi oscillations between the (2p)-(1s) states.